1. Technical Field
The present invention is generally related to a system and method for detecting markers on a PCB (printed circuit board) and, in particular, to a system and method for detecting markers on a PCB using an image processing technique based on histogram and template features.
2. Description of Related Art
Currently, electronic manufacturing is one of the major application fields of machine vision systems. In particular, before electrical components are placed on a PCB, the board and the components are inspected thoroughly in order to achieve high yield and to reduce repair costs. After inspection, defective circuits are marked so that they can be sorted from non-defective circuits and terminated from further processing.
Various types of markers are implemented by manufacturers in their manufacturing processes. For instance, as shown in FIG. 1a, one type of marker that is used to mark a defective circuit is a laser graving (i.e., a cross). In addition, FIGS. 2a and 3a illustrate a small ink spot and a large ink spot marker, respectively. FIG. 3a illustrates a large ink spot marker that covers the entire field of view. Further, FIG. 4a illustrates a circuit that is marked by a drill hole. FIGS. 1b, 2b,3b, and 4b are images of corresponding circuits that are not marked, thus, indicating non-defective (“good”) circuits. The availability and use of such varying marker types can render a marker recognition process inefficient if the type of marker used is not particularly compatible with the recognition software.
Furthermore, there are several types of manufacturing processes that are implemented in electronic manufacture, which can limit the type of marker recognition process used. For instance, with one type of manufacturing process, the board is first calibrated (i.e., the exact position is detected and corrected for) before the marker is recognized. In another process, the marker is detected prior to calibration. Without prior calibration, the position variation can be large, which requires that the recognition algorithm used be robust with respect to shifts and small rotations. In this case, the use of large markers that cover the whole field of view (either ink spots or adhesive patches) is typically required.
Accordingly, a system and method for recognizing markers, which is robust and operatively compatible with a broad range of marker types, is highly desirable.